Measuring and analysing the directional spectra of ocean waves.

Abstract

The importance of directional wave information has been recognised for a long time. However, for decades good measurements of directional spectra were limited almost exclusively to special research campaigns. Collecting directional wave climates from various sites and putting directional wave data to operational use were not a common practice. The situation has now improved. Many new instruments and analysis techniques have been developed for both in situ implementation and for remote sensing. It has become possible to extend the use of directional information over a much wider range, including many practical applications. This has opened up new possibilities. But it has also highlighted the difficulties associated with the directional spectrum. None of the present instruments can provide all the data that is required to calculate the directional spectrum in a robust way. Clever analysis techniques have been introduced that use physical and mathematical constraints to extract the directional spectrum from the limited data available. Each instrument and each analysis method has its own advantages and shortcomings, and the results are not necessarily comparable. If one wants to make use of the directional information, one needs an understanding of the properties of the instruments and the methods as well as the differences between them. COST action 714, supported by the European Commission, had the objective to promote the development of measurement techniques and the use of directional wave measurements. The action was launched in 1996, and, in the end, representatives of ten European countries (Belgium, Finland, France, Italy, Germany, The Netherlands, Norway, Portugal, Spain, United Kingdom) participated in it. Within the action a working group was created with the title Intercomparisons of Spectral Properties of Surface Waves for the purpose of improving the understanding of measurement techniques, methods of analysis, and comparisons between different instruments. The working group decided to publish its work in the form of a book describing the theory of directional wave spectra and the various instruments and analysis methods used to measure them, as well as comparisons between different instruments. The aim of the book is to give a comprehensive and up-to-date review of the instruments and methods of analysis available today for measuring the directional spectrum of ocean waves. In addition to the texts by members of the working group, the book contains contributions by other ocean wave experts.